Controlled angle viewing screens by interference techniques

ABSTRACT

A hardened gelatin film on a transparent substrate forms a rear projection viewing screen providing high efficiency and uniformity of illumination with predetermined scattering characteristics over a relatively wide field of view. This is accomplished by the presence within the gelatin film of a scattering pattern corresponding to a certain three-dimensional interference pattern recorded therein. This certain interference pattern results from the exposure of the gelatin film, after light sensitization thereof, with substantially coherent light which has passed through a diffusing medium having a given cross sectional area which is spaced a given distance from the film, followed by the development of the exposed film by a process which includes rapid drying of the film after immersion thereof.

United States Patent 1 1 1 3,909,111

Meyerhofer 1 1 Sept. 30, 1975 1 CONTROLLED ANGLE VIEWING SCREENS BY INTERFERENCE TECHNIQUES Dietrich Meyrhofer, Princeton, NJ.

Assignee: RCA Corporation, New York, NY.

Filed: Feb. 27, 1974 Appl. No.: 446,417

Inventor:

US. Cl. 350/117; 96/383; 96/116 Int. Cl. G03B 21/56 Field of Search 350/117, 127; 96/27, 38.3,

References Cited UNITED STATES PATENTS 11/1915 Morris 96/116 1/1929 Comstock 96/1'16 12/1944 Jelley et al 350/117 4/1974 Rotz et al. 350/117 OTHER PUBLICATIONS Hologram Formation in Hardened Dichromated Gelatin Film, by L. H. Lin, Applied Optics, Vol. 8, No. 5, pp. 963966, May 1969.

Primary E.\'aminerMonroe H. Hayes Assistant E.\'aminerAlan Mathews Attorney, Agent, or FirmEdward J. Norton; George J. Seligsohn [57] ABSTRACT A hardened gelatin film on a transparent substrate I forms a rear projection viewing screen providing high efficiency and uniformity of illumination with predetermined scattering characteristics over a relatively wide field of view. This is accomplished by the presence within the gelatin film of a scattering pattern corresponding to a certain three-dimensional interference pattern recorded therein. This certain interference pattern results from the exposure of the gelatin film, after light sensitization thereof, with substantially coherent light which has passed through a diffusing medium having a given cross sectional area which is spaced a given distance from the film, followed by the development of the exposed film by a process which includes rapid drying of the film after immersion thereof.

5 Claims, 1 Drawing Figure U.S. Patent Sept. 30,1975 3,909,111

CONTROLLED ANGLE VIEWING SCREENS BY INTERFERENCE TECHNIQUES BACKGROUND OF THE INVENTION This invention relates to a viewing screen combining high efficiency and uniformity of illumination with predetermined scattering characteristics, andparticularly to such a screen formed through interference techniques. 1 I

In rear projection systems such as slide viewers and microfilm readers, an image is generally projected onto a diffusing screen where it is presented to the observer. Such a rear projection screen is generally viewed near the axis of the system. The diffusing screen should scatter the largest possible fraction 'of the-light towards the observer, i.e., have a high efficiency, thereby requiring a smaller quantity of illumination. The; screen must scatter the light not only with high efficiency, but also in such a manner that the intensity of the light reaching the observer from various points on the screen isas uniform as possible, i.e.', have predetermined scattering characteristics. Conventional screens, e.g., ground glass, do not scatter the light as desired Often, conventional screens have the further disadvantage of allowing a hot spot to appear near the center of the screen, causing difficulty to the observer. It would thereforebe desirable to develop a screen capable of overcoming the disadvantages of the prior art.

SUMMARY OF THE INVENTION A method of forming a rear projection viewing screen where the screen combines high efficiency and uniformity of illumination with predetermined scattering characteristics. A hardened gelatin film is prepared on a substrate and then sensitized to light. The gelatin film is then exposed by transmitting substantially coherent light through a diffusing medium and then allowing the light transmitted through the diffusing me dium to fall upon the film. The film is then developed.

BRIEF DESCRIPTION OF THE DRAWING The drawing is an illustration of one form of exposure geometry capable of forming a viewing screen combining high efficiency and uniformity of illumination with predetermined scattering characteristics through the method of the present invention.

DETAILED DESCRIPTION Referring initially to the drawing, one form of exposure geometry for forming a rear projection viewing screen having the desired optical properties through the method of the present invention is designated generally as 10. The exposure geometry includes a source 12 of substantially coherent light, e.g., a He-Cd laser, and a diffusing medium 14 of a material having desired scattering characteristics, e.g., ground glass able to scatter in all directions. A lens 13 can be provided to focus the light on the diffusing medium 14. A hardened sensitized gelatin film 16, such as a dichromated gelatin, including ammonium, potassium and sodium dichromated gelatin, is affixed to one surface of a transparent substrate 18, e.g., glass, plastic, etc. It is preferable that ammonium dichromated gelatin be utilized as the film 16, as ammonium dichromate is a well known and easily available photosensitive material. The substrate 18 is positioned wherein the light from the light source 12 first passes through the diffusing medium 14 and then falls incident upon the gelatin film 16 as shown in the drawing.

In the method of the present invention, a hardened .gelatin film 16 is first prepared on the substrate 18 with a thickness preferably in the range of 6 to 20 microns.

The gelatin film 16'can be prepared on the substrate 18 through the method described by H. Lin in App. Optics 8, 963 1969), in an article entitled Hologram Formation 'in Hardened Dichromated Gelatin Films," although other available methods would be equally successful. According to the method of Lin, a 7% (by weight) solution of gelatin, such as USP powdered gelatin, commerciallyavailable from J. T. Baker Corporation, is poured over aglass plate, and the excess solution is permitted to run off the plate. After the film is -dry, it is hardened by soaking in a photographic fixer containing a hardener for-approximately 3 to 5 minutes, followed by washing in water. By varying the concentration ofthe gelatin solution and the use of multiple coatings, a film thickness inthe range of approximately '1 to 20 microns can be obtained. The hardened gelatin film 16 is then sensitized to light,'e.g.,*soaked in a solution of the dichromate, e.g., ammonium dichromate, as described by Lin in the previously mentioned article. r

The hardened, sensitized gelatin film l6-is now ready to be exposed. Exposure can successfully occur as shown in the drawing where the substantially coherent light emitting from the light source 12 falls upon the diffusing medium'14. The light from the light source 12 passes through the diffusing medium 14 and then falls upo'h the sensitized gelatin film 16. Although the phenomena is not clearly understood, it is believed that the light falling upon the hardened, sensitized film 16 interferes in the hardened, sensitized gelatin film 16 in such a way as to form interference planes which are randomly oriented in space except that they are all close to perpendicular to the plane of the gelatin layer 16. It is believed that a further small, localized, hardening occurs in portions of the hardened gelatin film 16 due to increased crosslinking as a result of changes in the state of the chromate ion, as the photons are absorbed. The gelatin film 16 is spaced :1 distance d from the diffusing medium 14. The diffusing medium 14 includes an area, having a linear dimension W of illumination due to the light emitted from the light source 12. The linear dimension W of illumination on the diffusing medium 14 subtends an angle a on the hardened, sensitized gelatin film 16. From the exposure geometry of the drawing, it is therefore apparent that the angle a is a function of the distance d and the linear dimension W. The diffusing medium 14 and the hardened, sensitized gelatin film 16 are positioned wherein the relation of the linear dimension W to the distance a is such that the angle a is preferably in the range of 15 to 40, with approximately 20 producing a viewing screen having the optimum scattering characteristics. The magnitude of the linear dimension W can be varied by employing shutters 20 to permit the light from the light source 12 to reach only a selected portion of the diffusing medium 14 while the distance d is varied by moving either the diffusing medium 14 or the gelatin film 16.

The exposed gelatin film 16 is then developed. The exposed film 16 is first washed in water, dehydrated in isopropyl alcohol, and then dried for about 30 minutes in an atmosphere of about relative humidity, followed by further drying at to relative humidity or at elevated temperatures, e.g., 100C, whereupon the exposure becomes permanent. It is believed that washing in water causes the exposed gelatin layer to expand and that the rapid drying rips apart the gelatin, thereby producing minute cracks where the gelatin layer was not exposed. The drying can be performed with greater reproducibility if the developed film 16 is pulled very slowly from the isopropyl alcohol with a stream of hot air simultaneously directed at the film 16 near the alcohol interface.

The light incident upon the diffusing medium 14 need not be collimated, but may be somewhat diverging from a point source, The incident light must be coherent to such an extent that all the light impinging on a given point on gelatin film 16 has a fixed phase relationship. The coherency requirement depends on the directionality of the diffusing medium 14 and the distance d. The distance d can be made as small as desired,

even to the point where the diffusing medium 14 contacts the gelatin film 16 wherein the coherency requirement is no serious restriction. Exposure can be made with the He-Cd laser or with various available high pressure mercury arc lamps as the light source 12. In the former case, the light emitted is highly coherent, thereby allowing the diffusing medium 14 to be separated by a long distance (I from the gelatin film 16.

The nature of the diffusing medium 16 becomes less important when the diffusing medium 14 is separated by a long distance 1 from the gelatin film 16 since the divergence of the light exposing the film 16 is determined by the angle a. If a mercury lamp is employed as the light source 12, spatial filtering through a pinhole can make the light suitably coherent. The use of mercury lamps would be desirable as they produce collimated light which is more uniform in intensity than that produced by the typical He-Cd laser. Furthermore, mercury lamps are less expensive and less bulky than laser light sources.

Thus, the method of the present invention provides a method of forming a rear projection viewing screen, combining high efficiency and uniformity of illumination with predetermined scattering characteristics. A screen produced by the method of the present invention has properties that are completely symmetric to the plane of the diffuser, and-about a line perpendicular to it.

I claim:

1. A rear projection viewing screen combining high efficiency and uniformity of illumination with predetermined scattering characteristics over a relatively wide field of view about the normal to said screen, said screen comprising:

a. a transparent substrate, said substrate having a pair of opposing surfaces, and

b. a hardened gelatin film on one of said surfaces of said substrate, said gelatin film exhibiting a scattering pattern corresponding to a certain threedimensional interference pattern recorded therein, the distribution of regions of constructive and destructive interference of said recorded certain interference pattern being distributed substantially randomly with respect to the plane of said screen and in determined preferred orientations with respect to the normal to said screen, whereby said distribution of said recorded certain interference pattern predetermines said scattering characteristics of said screen.

2. A viewing screen in accordance with claim 1 wherein the thickness of said gelatin film is in the range of 6 to 20 microns.

3. A viewing screen in accordance with claim 2 in which said gelatin film is a dichromated gelatin.

4. A viewing screen in accordance with claim 3 in which said film includes ammonium dichromate.

5. A viewing screen in accordance with claim 1, wherein said certain recorded interference pattern results from the exposure of said gelatin film, after light sensitization thereof, with substantially coherent light which has passed through a diffusing medium having a given cross sectional area which is spaced a given distance from said film, and the development of said exposed film by a process which includes rapid drying of said film after immersion thereof, said preferred orientations being determined by the size of the dimensions of said given area and the size of said given distance.

l l l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 909, 111

DATED September 30, 1975 INVENTOR(S): Dietrich Meyerhofer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title should read -REAR PROJECTION VIEWING SCREEN PROVIDING UNIFORMITY OF ILLUMINATION OVER A RELATIVELY WIDE FIELD OF VIEW-- Signed and Sealed this tenth Day Of February 1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'larenrs and Trademarks UNITED STATES PATENT OIIFICE CERTIFICATE OF CORRECTION PATENTNO. 3,909,111

DATED September 30, 1975 |NvENTOR(S): Dietrich Meyerhofer' It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title should read --REAR PROJECTION VIEWING SCREEN PROVIDING UNIFORMITY OF ILLUMINATION OVER A RELATIVELY WIDE FIELD OF VIEW-- Signed and Scaled this tenth Day Of February 1976 [SEAL] A trest:

RUTH C. MASON C. MARSHALL DANN Arresting Offirer Commissioner of Patents and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3, 909 111 DATED September 30, 1975 INVENTOR(S): Dietrich Meyerhofer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Title should read -REAR PROJECTION VIEWING SCREEN PROVIDING UNIFORMITY OF ILLUMINATION OVER A RELATIVELY WIDE FIELD OF VIEW-- Signed and Sealed this tenth Day Of February 1976 [SEAL] A! test:

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Patents and Trademarks 

1. A REAR PROJECTION VIEWING SCREEN COMBINING HIGH EFFICIENCY AND UNIFORMITY OF ILLUMINATION WITH PREDETERMINED SCATTERING CHARACTERISTICS OVER A RELATIVELY WIDE FIELD OF VIEW ABOUT THE NORMAL TO SAID SCREEN, SAID SCREEN COMPRISING: A. A TRANSPARENT SUBSTRATE, SAID SUBSTRATE HAVING A PAIR OF OPPOSING SURFACES, AND B. A HARDENED GELATIN FILM ON ONE OF SAID SURFACES OF SAID SUBSTRATE, SAID GELATIN FILM EXHIBITING A SCATTERING PATTERN CORRESPONDING TO A CERTAIN THREE-DIMENSIONAL INTERFERENCE PATTERN RECORDED THEREIN, THE DISTRIBUTION OF REGIONS OF CONSTRUCTIVE AND DESTRUCTIVE INTERFERENCE OF SAID RECORDED CERTAIN INTERFERENCE PATTERN BEING DISTRIBUTED SUBSTANTIALLY RANDOMLY WITH RESPECT TO THE PLANE OF SAID SCREEN AND IN DETERMINED PREFERRED ORIENTATION WITH RESPECT TO THE NORMAL TO SAID SCREEN, WHEREBY SAID DISTRIBUTION OF SAID RECORDED CERTAIN INTERFERENCE PATTERN PREDETERMINES SAID SCATTERING CHARACTERISTICS OF SAID SCREEN.
 2. A viewing screen in accordance with claim 1 wherein the thickness of said gelatin film is in the range of 6 to 20 microns.
 3. A viewing screen in accordance with cLaim 2 in which said gelatin film is a dichromated gelatin.
 4. A viewing screen in accordance with claim 3 in which said film includes ammonium dichromate.
 5. A viewing screen in accordance with claim 1, wherein said certain recorded interference pattern results from the exposure of said gelatin film, after light sensitization thereof, with substantially coherent light which has passed through a diffusing medium having a given cross sectional area which is spaced a given distance from said film, and the development of said exposed film by a process which includes rapid drying of said film after immersion thereof, said preferred orientations being determined by the size of the dimensions of said given area and the size of said given distance. 